1. Field of the Invention
The present invention relates to nuclear reactor cladding and more specifically the invention relates to coatings to nuclear reactor cladding which provide wear- and chemical-resistance.
2. Background of the Invention
According to EPA statistics, emissions from energy-related activities account for 85.5 percent of the United States' total greenhouse gas emissions. Approximately 80 percent of these energy-related emissions were from the combustion of fossil fuels.
Nuclear energy offers a viable alternative to fossil fuels in that it is comparatively clean. Unfortunately, the public is wary of nuclear energy given the high-profile containment breaches of Three Mile Island in 1979, Chernobyl in 1986, and Fukushima, Japan in 2011. In order to change the public perception of nuclear energy, the safety of nuclear reactors must be assured.
A typical light water reactor (LWR) utilizes the energy produced from fission reactions to heat water or steam infiltrating the reactor core. A simplified representation of a nuclear reactor core can be seen in FIG. 1A. The reactor core 20 contains a plurality of control rods 22 and fuel rods 24. The control rods 22 are made of highly neutron absorbent materials, such as silver, indium, hafnium, boron, and cadmium. The fuel rods 24 comprise fissile material encapsulating in a cladding. Depending on the power production needs, the control rods 22 are inserted or removed so as to moderate the flux of neutrons available for fission reactions.
The energy produced in the fission reactions heats the coolant in the reactor core. For example, in a boiling water reactor, the steam produced in the reactor core heats a secondary loop which in turn spins downstream turbines to produce mechanical energy or motion.
In a pressurized water reactor, the reactor vessel is pressurized to prevent the heated water from turning into steam. The pressurized water travels through a heat exchanger to vaporize clean water (in a secondary water loop) to steam. This steam powers downstream turbines. The mechanical energy from the turbines turns a generator, which results in the production of electricity. In both boiling water and pressurized water reactors, the steam exiting the turbines is cooled by a condenser before being rerouted through the secondary loop in the reactor core.
Fuel cladding is the most important of the three radioactivity barriers existing in nuclear reactors. (The other two are the pressurized vessel and reactor containment structure.) As long as the fuel cladding remains intact during an accident, no threat to public health and safety is expected.
FIG. 1B is a detailed view of a section of one of the fuel rods 24. As can be seen in FIG. 1B, the fuel rods 24 contain the fissile material, typically in the form of fuel pellets 26. In a typical LWR, the fuel pellets 26 are a sintered oxide, predominantly uranium dioxide. Surrounding the fuel pellets 26 is a cladding layer 28, which is typically made of zirconium or a zirconium alloy.
The cladding 28 is primarily responsible for enclosing, encapsulating and otherwise physically isolating the radioactive fuel from areas exterior from the cladding. For this purpose, zirconium has exceptional properties, including low-neutron absorption, high hardness, ductility, and corrosion resistance. Zirconium alloys typically contain greater than 95% zirconium with the balance being other metals, such as tin, niobium, iron, chromium, and nickel. However, zirconium's properties degrade as temperatures reach approximately 800° C., which can happen after a loss-of-coolant-accident (LOCA). The zirconium will react with the coolant water or steam and form an oxide, which produces hydrogen gas. Not only does the presence of hydrogen gas increase the risk of an explosion, but it also causes hydrogen embrittlement. The hydrogen embrittlement leads to blistering and cracking of the cladding 28, providing openings through which radioactive materials can escape.
The water or steam in the reactor core is part of a closed loop (the primary loop) that does not mix with, or contaminate, the water circulating in the condenser (the secondary loop). Maintaining this separation of primary loop water and secondary loop water is critical to the operation of a nuclear power plant. However, contamination of the clean water can happen through a variety of albeit dramatic circumstances. Meltdown, in particular, can lead to a severe breach of containment. A meltdown occurs when a reactor core component melts, exposing uncontaminated coolant to radioactive contaminants.
A core meltdown takes place in six stages:                1. The core is “uncovered,” which means that the fuel rods are no longer being covered by coolant. Absent coolant, the fuel rods begin to heat up. Core exposure can happen through a LOCA.        2. The reactor core undergoes pre-damage heat up, whereby temperatures increase at a rate of 0.3° C./s to 1° C./s (degrees Celsius per second).        3. When temperatures reach approximately 1100 K, the zirconium cladding around the fuel rods will begin to expand (i.e., balloon out) and eventually burst. Ballooning takes place because the pressure inside the fuel rod is much greater than outside the fuel rod. As temperature is increased the requisite pressure differential and time to bursting are significantly decreased. Because the fuel rods are closely bundled, the ballooning can lead to a blockage of coolant flow between the rods, further exacerbating the overheating. If the zirconium cladding bursts, then radioactive materials may escape.        4. At approximately 1500 K, the zirconium cladding will undergo rapid oxidation by reacting with the ambient high pressure steam. The oxidation of zirconium will strip the oxygen from the steam, producing hydrogen gas. Excess hydrogen gas causes embrittlement and hydrogen explosions. The disaster in Fukushima, Japan involved a hydrogen explosion that blew the roof off the reactor buildings.        5. As temperatures continue to rise, components of the core will become molten and begin to flow to the lower region of the fuel rods. At this stage, the uranium dioxide fuel pellets, or any of the other fuel materials, will have at least partially dissolved into the zirconium cladding.        6. The final stage of meltdown occurs when the molten reactor core components, including melted fuel pellets, forms a lava-like mixture, referred to as corium, and flows to the bottom of the reactor vessel, or the lower plenum. Water will still be present in the lower plenum, and the flow of the corium into the water will generate large amounts of steam, potentially triggering a steam explosion. Further, the corium can melt through the lower plenum, resulting in the escape of radioactive material.        
A need exists in the art for increasing the temperature and chemical resistance of existing nuclear fuel cladding materials. The method should result in claddings resistant to ballooning, bursting and rapid oxidation. The method should yield cladding which does not require decades of certification but rather can be put into service in a short period of time and with no retrofit to existing reactor cores.